The present invention relates to alarm systems having multiple remote devices in communications with a receiver/control unit wherein the receiver/control unit comprises one or more antennas; and in particular to such alarm systems wherein the receiver sensitivity is modified during the installation of a remote device and during the reception of a supervision signal from a remote device, in order to ensure there is adequate margin between the remote device and the receiver/control unit during normal alarm signal transmission operations.
Contemporary radio frequency (RF) wireless security systems, such as those utilizing the ADEMCO 4281 or 5881 receiver, often use receiver antenna diversity, whereby signals from different locations are equally detected. The information transmitted from remote devices typically describes the state of various sensors, such as smoke, motion, breaking glass, shock and vibration detectors; door, window and floor mat switches; etc. These remote device sensor products are designed to be low in cost and are typically send-only devices, as opposed to send/receive, or transceiver, devices which are significantly more expensive.
As taught in U.S. Pat. No. 4,754,261 to Marino, during alarm system installation, transmission of signals from the remote devices to the receiver/control unit at a reduced sensitivity allows the installer to check the transmission signal margin of the remote devices. If the receiver/control unit recognizes a message from each remote device at a reduced sensitivity of the order of approximately 12 dB, then there will be adequate signal margin during normal operation. That is, if the radio environment changes during normal operation, due to movement of furniture etc., the alarm signal transmissions have sufficient signal strength to be received by the receiver/control unit in this changed, adverse condition.
In addition, in order to meet basic regulatory agency requirements, the remote devices are required to transmit periodic supervision transmission signals in order for the receiver/control unit to monitor proper operation of all of the remote devices in a given system. The supervision signal (as well as an alarm signal) has a unique identification code embedded in its data message, which serves to identify to the receiver/control unit which particular transmitting remote device has sent that supervision (or alarm) message. For life safety applications, the RF wireless system must also comply with more stringent regulations, such as the Underwriters Laboratories regulation UL864. This regulation additionally requires that the supervision signal be reduced in transmission power level below that of the alarm (normal, non-supervision) signal transmission by a minimum of approximately 3 dB or by other equivalent means, to ensure that the alarm signal has an effective power margin over that of the periodic supervision signals from each remote transmitter device in the system.
Employing transmitter-only products that would accurately transmit an alarm signal at the maximum allowable level and then reducing that power level during the periodic supervision signal transmission or during alarm system installation, would add significant additional cost to each transmitter product. Therefore, the sensitivity reduction desired in installation as well as supervision may be achieved at a point in the receiver which is "post detection" by means of a simple thresholding scheme as set forth in co-pending U.S. application Ser. No. 08/650,292. Such a thresholding scheme is often very cost effective, but may lead to pulse distortion which may have unforeseen side effects in the decoding process. It would be advantageous to introduce a reduction in sensitivity at a "pre-detection" point at the receiver, where the reduction in sensitivity truly effects the system signal-to-noise ratio.
It is therefore an object of the present invention to provide a method for ensuring that an adequate signal margin exists between the receiver/control unit and the remote devices during alarm system installation.
It is a further object of the present invention to provide an alarm system with a supervision mode, wherein a reduced power signal is effectively generated and processed without the remote device being altered.
It is a further object of the present invention to provide a method of impairing the receiver sensitivity during alarm system installation and during a supervision transmission sequence.
It is a further object of the present invention to provide a method of reducing the receiver sensitivity at a "pre-detection" point within the receiver.